Conduit or pipe for transporting fluids, containing underground cables, and the like has for many years been extruded from synthetic resin such as poly-vinyl chloride (PVC). More recently, such pipe has been made lighter and more economically by forming its walls to have a layered construction with an intermediate layer of a relatively inexpensive foamed synthetic resin sandwiched between inner and outer walls of a more resilient and corrosion resistant resin. Such pipes are commonly referred to as triple-wall conduit.
A number of assemblies commonly known as coextrusion heads for extruding triple-wall conduit have been developed and employed in the pipe extrusion industry. Examples of such heads are disclosed in U.S. Pat. No. 4,125,585 of Rosenbaum, U.S. Pat. No. 4,731,002 of Spence, and U.S. Pat. No. 4,783,299 of Prevotat. These devices usually comprise a stainless steel or other metal body that defines a number of internal passageways. The passageways are configured to receive flows of malleable synthetic resin from external extruders, shape them into generally concentric annular shells as they move trough the head, and bring the shells into mutual confluence to define a triple-wall sandwiched flow. Once formed, the sandwiched flow typically passes from the coextrusion head into a conventional sizing die, which condenses the wall thickness and sizes the flow to a desired diameter forming the finished conduit.
While such devices have proven somewhat successful for production of coextruded triple-wall conduit, they nevertheless have been plagued with numerous problems and shortcomings inherent in their respective designs. In the Rosenbaum coextrusion head, for example, internal passageways exhibit numerous sharp turns, discontinuous junctions, and narrow portions that must be negotiated by the resin flows thereby introducing resistance to the flow and causing malformations in the resin.
Further, webs that support an internal spider and accommodate radially extending flow passageways are disposed in some of the flow paths of the prior art heads such that resin flowing through such flow paths must split, traverse the webs, and come back together at the trailing edges of the webs. Such flow splitting and recombination can introduce additional malformations and discontinuities in the resinous material of the flow.
In addition, radial passageways in Rosenbaum tend to intersect axial annular passageways at a single circumferential location such that resin moving into the annular passageway must first flow around its circumference before it can move axially therethrough. This tends to introduce even more malformations and discontinuities in the flow and ultimately in the finished conduit itself.
The Spence head, while offering some improvements over Rosenbaum, still has an internal spider supported in a flow passageway by radially extending webs and still has flow paths that exhibit sharp and discontinuous turns that must be negotiated by the resin flows. Many of the problems of Rosenbaum are therefore also inherent in the design of Spence. Other prior art coextrusion heads exhibit similar problems.
In addition to having the just described problems, prior art coextrusion heads also tend to be extremely difficult to disassemble and clean. This is because such disassembly must necessarily be carried out with resin in the passageways of the head. Obviously, the discontinuous passageways, internal spiders and their support webs greatly hinder disassembly since the resin extending through and around such structures tends to bind component parts of the head together.
Thus, a critical and heretofore unaddressed need exists for a coextrusion method and apparatus adapted for reliable production of consistently high quality triplewall conduit through minimization of resin flow resistance, elimination of sharp discontinuous passageway junctions, and elimination of spiders and their associated webs that obstruct flow passageways and degrade the continuity of resin flow therethrough. Such an apparatus should also be constructed for easy disassembly and cleaning when required, even with resin embedded in its flow passageways. It is to the provision of such a method and apparatus that the present invention is primarily directed.